1. Field of the Invention
This invention relates to a key telephone system.
2. Description of the Prior Art
When to make an intercomm. call from a key telephone set by depressing a calling key provided in a calling device or the like during communication with a CO line, it is the practice in the prior art to depress the calling key after depressing a holding key provided in the key telephone set to hold the CO line. However, this method involves two key depressing operations and hence is inconvenient and is likely to lead to an erroneous operation. To avoid such inconvenience, there has also been proposed a system of holding the CO line by depressing the calling key to mechanically actuate the holding key in ganged relation thereto, but this system has the defects of its complicated and bulky structure and low reliability.
Intercomm. individual calling in a key telephone system is usually achieved by dialing individual numbers respectively assigned to key telephone sets, but this method requires dialing for each call and hence is inefficient and inconvenient especially when to frequently call from a key telephone set receiving an incoming call from a CO line. To correct such a defect, there has been proposed a system of calling a called party by depressing a corresponding one of calling keys, each provided for one key telephone set, instead of dialing. Also with this method, however, it is necessary to continue calling until the called party answers. For example, in a case where the receiving key telephone set transfers the incoming call signal from a CO line to a called key telephone set, calling must be continued until the cable party answers.